This invention relates to auxiliary fluid power steering systems for motor vehicles having reaction force generating means.
Motor vehicles are often provided with a single hydraulic pressure generating unit supplying pressurized fluid to several independent fluid pressure operated systems such as power steering and braking systems, or systems for regulation of a suspension level that require a variable volumetric supply of fluid. In order to maintain the hydraulic pressure generating unit relatively small in dimension despite the different fluid pressure operated devices serviced thereby, pressure storage facilities is provided in order to store fluid under pressure during periods of low fluid consumption. Accordingly, a relatively high storage pressure is maintained which is about twice as high as the highest operating pressure required for normal steering purposes in the fluid power steering system. The fluid power steering system is therefore constructed in such a way as to block flow from the high pressure source to the fluid reservoir in the neutral position of the steering wheel.
Auxiliary power steering systems of the foregoing type as disclosed in U.S. application Ser. No. 357,603, filed May 7, 1973, aforementioned includes two control valve pistons and two reaction pistons, each pair being axially aligned in symmetrical relationship to the valve actuators driven by the hand steering wheel. In the deflected positions of the hand steering wheel, pressurized fluid is fed to one of the operating chambers of a servomotor while the other operating chamber is connected to the low pressure reservoir. The reaction pistons function to resist movement of the hand steering wheel with a proportional reaction force by supply of operating pressure thereto from the servomotor. The reaction pistons are arranged, however, to enable the reaction force to rise no further than a certain maximum value by use of a coaxial force limiting piston acted upon by the storage pressure of the pressurized source. In the foregoing type of reaction force limiting arrangement, the reaction force limiting function is accomplished hydraulically only. However, in order to obtain sufficiently large reaction forces at the hand steering wheel, the reaction and force limiting pistons must have relatively large diameters. In utilizing large diameter pistons, jamming often occurs because of piston guide defects. Further, a relatively large sealing surface is associated with large diameter pistons which is a serious disadvantage in connection with the force limiting pistons. In another type of reaction force limiting arrangement as disclosed in my prior U.S. Pat. No. 3,855,904 aforementioned, a pre-stressed spring is utilized to determine the maximum level of reaction force that must be overcome by the driver on the hand steering wheel. The latter type of reaction force control arrangement also involves a relatively expensive construction.
It is therefore an important object of the present invention to provide a reaction force limiting arrangement for fluid power steering systems associated with a fluid pressure storage supply which will insure faultless behavior during operation by apparatus of relatively small spatial dimension.